Maybe there isn't enough downside built into Pfizer's (NYSE:PFE) shares.

In a precursor to what the next few years will bring, sales of Pfizer's best-selling drug, Lipitor, slipped significantly during the second quarter. Total sales of the blockbuster cholesterol drug fell 13% on a worldwide basis, due primarily to a 25% plummet here in the United States.

The higher-than-expected drop is concerning because Lipitor accounted for more than one-fourth of sales for the quarter and is expected to bring in much-needed cash flow until it loses patent protection once and for all -- sometime around 2010. In other words, Pfizer needs time to replace billions of dollars in lost revenue with new drugs, and time may be running out faster than expected.

If it's any consolation, CEO Jeffrey Kindler stated that "the entire management team is working tirelessly to identify ways to improve the performance and outlook for Pfizer." However, investors appear increasingly skeptical after today's second-quarter earnings release, as the company is also grappling with the loss of Zoloft and Norvasc to generic competition from the likes of Barr Pharmaceuticals (NYSE:BRL), Teva Pharmaceutical (NASDAQ:TEVA), and Mylan Labs (NYSE:MYL)

The combined challenges contributed to a 6% fall in total sales for the quarter, and reported net income fell an even more sickening 48%. Pfizer has begun to release adjusted earnings results to try to give investors a better indication of the operating picture, since it is also taking numerous charges to reduce costs and become leaner to prepare for L-day, or when Lipitor revenue mostly goes away. But even those tweaked results show a 16% bottom-line decline.

The recent quarter proved more trying than originally anticipated, but management is sticking to the outlook it provided for this year and expects to report $2.31-$2.45 in adjusted earnings per share in 2008.

Additionally, it expects to generate as much as $13 billion in operating cash flow this year and up to $19 billion next year. That's somewhere around $1.85 per share this year and $2.71 next year, leaving plenty of room to buy back shares, enhance the 4.5% dividend yield, and develop or acquire new drugs to get sales back in a positive direction.

It won't be easy and there is no guarantee that Pfizer will grow its way out of over-reliance on a dying Lipitor franchise, but I'm happy collecting a hefty dividend for the next couple of years and hanging around at a low cash flow multiple to see if Pfizer will become the next Merck (NYSE:MRK) or Wyeth (NYSE:WYE) by eventually overcoming product-related adversity.       

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Fool contributor Ryan Fuhrmann is long shares of Pfizer but has no financial interest in any other company mentioned. Feel free to email him with feedback or to discuss any companies mentioned further. The Fool has an ironclad disclosure policy.